What is the function of a feeler gauge?

Question: What is the function of a feeler gauge?

A feeler gauge is a tool used to measure gap widths. Feeler gauges are mostly used in engineering to measure the clearance between two parts. They consist of a number of small lengths of steel of different thicknesses with measurements marked on each piece. They are flexible enough that, even if they are all on the same hinge, several can be stacked together to gauge intermediate values. Feeler gauges, sometimes called thickness gauges or feeler gages, are mechanical measurement instruments that are used to provide a precise reading of the gap that exists between two parallel surfaces, such as the clearance between two machine parts or elements.